The present invention relates to a phase modulator for the modulation of the phase of circular polarised light which interacts with the phase modulator. The present invention further relates to a display for the presentation of two-dimensional and/or three-dimensional image contents.
According to S. Pancharatnam, Proc. Ind. Acad. Sci., p. 137, 1955, the phase of light can be modulated in that circular polarised light is used and that a λ/2 plate is rotated in its plane. This is shown schematically in FIG. 1. Circular polarised light falls on a λ/2 plate. The direction of rotation of the circular polarised light changes. In addition, a phase occurs which depends on the angle of the optical axis of the λ/2 plate in the plane. If the λ/2 plate is turned by the angle φ (lower part of FIG. 1), then the phase at the exit will change by the angle 2φ. The phase change is thus twice the rotation angle of the λ/2 plate. Consequently, a phase modulation or phase change of 360 degrees (2π) is achieved by turning the λ/2 plate by 180 degrees.
Instead of mechanically rotating a λ/2 plate, it is possible that in a light modulator which is based on liquid crystals (LCs) the long axis of the LC molecules is turned, for example as induced by the application of an electric field.
However, when doing so, nematic LCs typically only react on the absolute value and not on the sign of the applied voltage. The LC molecules can rotate between 0 and maximum 90 degrees only given a certain surface orientation of the LC molecules and an electric field in the pixel plane—as in an in-plane switching liquid crystal mode (IPS LC mode)—where the electric field is applied at an angle relative to the surface orientation and where this angle can be up to 90 degrees. Smectic LCs, such as used in for example in a polarisation-shielded smectic liquid crystal mode (PSS LC mode), will change their direction of rotation as the sign of the field changes. However, +90-degree rotation or −90-degree rotation of the LCs as such is not possible. In contrast to nematic LCs, smectic LC molecules are arranged in layers, and a 90-degree rotation of the individual molecules would not be possible while maintaining this layered structure. The desired angular range of LC molecule rotation of 180 degrees is thus not achievable using a conventional LC mode.
Document DE 10 2009 045 125.0 and the international patent application PCT/EP2010/064504 describe a solution for this problem, where the angular range of LC molecule rotation is enlarged by combining a switchable surface alignment with one of said LC modes. The disadvantage of that solution is that it requires more elaborate and costly manufacturing and control processes. This is because a special alignment layer, which involves other than the standard materials used in display panel production today, must be applied to a substrate when manufacturing a spatial light modulator (SLM). When addressing the SLM, it may be necessary to generate separate signals for switching the surface orientation and for direct control of the LC molecules. This may further require faster signal transmission when addressing the pixels of an SLM if the two signals are needed one after another in order to set an individual phase value by the combination of the two signals.
WO 2008/104533 A1 and publications of similar content, such as conference documents of Eurodisplay 2009 [1] and Imid 2009 [2], describe a hybrid aligned nematic LC mode (HAN). The LC molecules which are sandwiched between two substrates align perpendicular to one substrate surface, but parallel to the other substrate surface. This surface orientation is fix. Parallel orientation is typical for example in IPS or twisted nematic (TN) mode arrangements, whereas perpendicular orientation is typical in vertical alignment (VA) mode arrangements. The two substrates require different alignment layers, but both types can be made with standard procedures which are known in the LCD industry.
In the most simple theoretical model, the LC molecules are commonly referred to as ‘rigid rods’. Deviating from this simple assumption, LC molecules can for example also have a curved, ‘banana-shaped’ form or a clubbed, ‘pear-shaped’ form. While in two ideal rods a parallel and anti-parallel orientation are energetically equal, in the alternative shapes, in particular in the case of an induced deformation as in splayed or bent shapes, however, parallel orientations of the pear- or banana-shaped molecules are preferred relatively to anti-parallel orientations.
A deformation of the LC thus induces a polarisation in an LC material with a corresponding molecule shape. This is known as the ‘flexo-electric effect’. If there is a flexo-electric polarisation, then the LC molecules react specifically to the sign of an applied electric field.
In the HAN arrangement, such a deformation is induced caused by the different surface orientations of the LCs at the two substrate surfaces and by the elastic forces among the individual LCs (due to a continuous transition from parallel to perpendicular orientation across the thickness of the LC layer), so that a flexo-electric polarisation is generated.
If an in-plane field is applied, then the LC molecules, or their projection into the display plane (which is parallel to the surface of the SLM substrate), will rotate. Due to the flexo-electric polarisation, the direction of rotation of the molecules then depends on the sign of the voltage. Document WO 2008/104533 A1 describes arrangements where the electrodes are arranged as in an IPS display and arrangements where an additional base electrode is disposed on the same substrate, as in a fringe-field switching (FFS) display. Document WO 2008/104533 A1 further describes arrangements where in-plane electrodes or FFS electrodes are optionally disposed on the substrate with parallel orientation of the LC molecules or on the substrate with vertical orientation of the LC molecules. The former is described there as the embodiment for LC materials with positive Δε, the latter as the embodiment for LC materials with negative Δε.
According to document WO 2008/104533 A1, the purpose of these arrangements is to realise short response times for the two switching processes (switching on and switching off), because both of them are controlled by one field in that different signs of the voltage may be used. For amplitude modulation, linear polarised light is used, and the required LC molecule rotation angles only range from −45 degrees to +45 degrees.
In the electrode arrangement described in document WO 2008/104533 A1 and related publication [1], the same voltage is supplied to every other electrode, similar to an IPS arrangement. This means that positive and negative fields between two electrodes occur in alternate arrangement. Consequently, the direction of rotation of the LC molecules also alternates in small regions within a pixel. This does not matter in an amplitude modulator, because the amplitude modulation depends on the absolute value but not on the sign of the rotation angle. For phase modulation, however, this arrangement would not be suitable because different phase values would be realised in different regions within one pixel. The same applies to an FFS-style electrode arrangement. One direction of rotation would be realised in one half of the spaces between two electrodes of the grid, whereas a different direction of rotation of the LC molecules would be realised in every other space, as illustrated in FIG. 3 in publication [2]. Such an arrangement would again be suitable for amplitude modulation, but not for phase modulation.